Conventionally, as an inkjet printer, one having an inkjet printhead which discharges ink droplets, a main tank which stores ink to be supplied to the ink-jet printhead, and a subtank which holds the ink supplied from the main tank is known.
In an inkjet printer of this type, many ink supply mechanisms which supply ink to the ink-jet printheads have been proposed and put into practical use. To supply the ink to the inkjet printhead, the capillary force of the nozzle itself of the ink-jet printhead is utilized, and accordingly no external force of a pump or the like is usually required. Therefore, a mechanism which supplies ink with a pressure from a subtank (reservoir ink tank) to the inkjet printhead is not required except for a special case. To cause ink droplets to stably fly from the nozzle of the inkjet printhead, a very low negative pressure of (−) 30 [Pa] to (−) 2,000 [Pa] must be applied. This is a significant issue in designing the inkjet printer.
To realize this, many attempts have been made to provide a negative pressure generating mechanism to an ink reservoir apparatus having a reservoir ink tank. The structure of a conventional ink reservoir apparatus will be described with reference to the accompanying drawings.
FIG. 19 is a schematic view of the structure of an ink reservoir apparatus employing a spring bag scheme. As shown in FIG. 19, in this ink reservoir apparatus, a coil spring 222 is arranged in a bag 221 which stores ink 223 in order to generate a negative pressure. The elastic force of the coil spring 222 made of a metal or the like applies an expansion force that expands the bag 221 in directions of arrows S1 and S2, so that the ink 223 generates a negative pressure. In this ink reservoir apparatus, the ink 223 is supplied from a supply port 224 formed in the bag 221.
FIG. 20 is a schematic view of the structure of an ink reservoir apparatus employing a regulating-valve-added bag scheme. As shown in FIG. 20, in this ink reservoir apparatus, a pressure regulating valve 231 is provided to a housing 230 which covers a bag 221 storing ink 223. The pressure regulating valve 231 causes external air 233 to flow into the housing 230, to control the pressure caused by inner air 232 outside the bag 221, so that a negative pressure is generated in the ink 223 in the bag 221. As in these ink reservoir apparatuses, when the internal pressure of the soft, flexible bag 221 is to be controlled with some mechanism, generally, the number of components increases and the manufacturing cost increases. It is also technically difficult to manage generation of a negative pressure of about several hundred Pa. If a negative pressure generating mechanism is provided, the ability to hold ink that can be used may decrease. Furthermore, when the bag is thin, it has poor hermeticity. When ink is stored in the thin bag over a long period of time, the external air may enter the bag to expand it, or the ink in the bag may evaporate. Therefore, when a mechanism that generates a negative pressure is to be added to an ink reservoir apparatus using a bag while ensuring the reliability, many problems must be solved.
FIG. 21 is a schematic view of the structure of a currently mainstream ink reservoir apparatus employing sponge. As shown in FIG. 21, in this ink reservoir apparatus, a sponge 241 is arranged in a housing 240 having a vent hole 242 and supply port 243. The sponge 241 can hold ink with the capillary force of itself. Thus, a desired negative pressure can be ensured by only selecting the density of the sponge. This ink reservoir apparatus has a very simple structure and can be manufactured at a comparatively low cost if a commercially available sponge is used. This ink reservoir apparatus can be downsized. A negative pressure is generated regardless of a difference in posture of the ink reservoir apparatus.
A sponge manufactured by a general sponge manufacturing method, however, does not have a sufficiently high density, and must be used after it is compressed to a certain degree. Consequently, with the sponge scheme, the use efficiency of the ink of the sponge degrades, and generally the sponge can be filled with the ink to as low as about 70% the sponge volume.
Generally, when that portion of an inkjet printer with which the ink comes into contact is to be made of a metal, it is made of stainless steel, and when it is to be made of a resin material, it is made of polypropylene, polyethylene, a fluoroplastic, or the like. When the ink contact portion comes into contact with the ink, a trace amount of decomposed material or additive sometimes elutes to the ink. A commercially available sponge is often made of a urethane resin and has a comparatively low chemical stability. For this reason, in recent years, a sponge made of polypropylene which is chemically more stable has been employed.
Since a porous body such as a sponge comes into contact with the ink with a large area, it may chemically react with the ink, or its additional matter may dissolve in the ink. Then, a large amount of product produced from the ink often adversely affects a portion in the vicinity of the nozzle. Various types of ink are used to expand the use of the ink-jet printer, but the chemical stability of the sponge poses an issue. Accordingly, the composition of the ink must often be unavoidably changed to improve the chemical stability, while the physical characteristics are degraded.
Furthermore, an ink holding body manufactured by compressing a urethane resin sponge, as described above, or polypropylene or polyethylene fiber has a comparatively large compression distribution. When such an ink holding body is repeatedly refilled with the ink, its compression structure includes air bubbles, and its ink filling rate gradually decreases. This phenomenon is caused due to the following reason. When refilling the ink, the ink is filled in the dense portion of the ink holding body first because the dense portion has a comparatively large capillary force, while the ink is not filled in the sparse portion of the ink holding body. Consequently, air bubbles are left in the sparse portion to form air bubbles. Once air bubbles are generated, they tend to remain as they are even after the ink is drawn out. As refill is repeated, the size and number of air bubbles increase and the filling rate decreases.
FIG. 22 shows another arrangement having the same function as that of the sponge which serves to hold the ink and to generate the negative pressure. FIG. 22 shows an arrangement in which, in place of a porous body such as a sponge, a plurality of thin plates 251 provided in a housing 250 at gaps hold ink. The narrow gaps between the thin plates 251 are utilized as an ink reservoir 253 (for example, see Japanese Patent Laid-Open Nos. 4-179553 and 3-139562). In this arrangement, the ink reservoir 253 holds the ink and generates a negative pressure with the capillary force which is expressed by a classic expression h=2T cos θ/ρ gr. In this manner, an ink reservoir apparatus using the plurality of multilayered thin plates 251 has a comparatively simple structure and enables reliable size management that does not depend on a manufacturing method as with the sponge.
To extract the ink from the ink reservoir 253 reliably, however, another capillary body 255 must be arranged to desirably extend through the respective multilayered thin plates 251. The capillary body 255 must have a larger capillary force than that of the ink reservoir 253, resulting in an excessively large ink channel resistance. Therefore, when this ink reservoir apparatus is applied to a high-frequency inkjet printer which consumes a particularly large mount of ink and has many nozzles, while the ink is supplied, the dynamic resistance increases. Accordingly, sometimes the ink is not discharged from a supply port 252.
As described above, in the inkjet printer, an ink reservoir apparatus is sought for which is manufactured at an inexpensive cost, which is chemically stable against ink, which generates a negative pressure with a low ink channel resistance regardless of a difference in posture of the reservoir ink tank, and which supplies the ink to the inkjet printer stably.
In particular, in an inkjet printer which prints while refilling with ink a subtank which temporarily holds ink supplied from a main tank, as refill is repeated, the filling rate of the ink that can be refilled in the subtank decreases. This phenomenon is a critical problem.